A traditional and consolidated technology for forming plastic products is injection moulding of thermopolymers; this is the typical technological method, in which material in the fluid, liquid or semi-liquid state is introduced to into a permanent mould (die), thrust by an injection force.
Injection moulding briefly comprises a plastification and injection group in which the polymer is melted, and a die having a chamber with fixed walls which gives shape to the product, into which the fluid polymer is injected.
As a crude base material the injection machine uses plastic granules which is are made to pass internally of a cylinder by means of an endless screw (Archimedes screw). The melting process internally of the cylinder is performed by means of heat induced by electrical resistances and by friction generated by the motion of the endless screw internally thereof.
The melting or softening temperature (vitreous transition) depends on the type of material which is to be used; it normally varies from 160 degrees Celsius for low density polyethylene (LDPE) up to 300 degrees Celsius for polycarbonate (PC).
The chamber of the die keeps the internal surfaces still, which the material in the fluid state is injected into the chamber up to filling it completely, thus defining the form to be obtained, and enables rapid cooling of the molten plastic and performs the expulsion of the solid finished product using special mechanical means known as extractors.
The injection pressure to which the die cavity is subjected is usually in the order of 300-600 Kg/cm2. The dimensions of the die and the energy consumption thereof are strongly conditioned by the surface of the product to be moulded, and by the pressure applied during the injection stage, and are always relatively very high.
This technology is distinguished by its relatively very high costs for the dies and the injection machines.
Other known technologies are illustrated in United States publications US-A-2003 051853 and U.S. Pat. No. 5,193,407.
A different and more recent technology in which the present invention is applied comprises compression-forming of thermoplastic material products placed in the die matrix.
The compression penetration of the punch (male part) into the matrix chamber (female part) is performed after having inserted therein a batch of plastic material in the solid state (at ambient or pre-heated temperature), transformed into relatively small particles which make it sufficiently fluid, which batch is softened (possibly melted) internally of the die by contact thereof with the matrix and the punch, which are both heated by contact with two opposite heated plates (by electrical resistances), belonging to a press which compresses the plastic material placed in the matrix, with thrusts of relatively high entity, in the order of 50-100 kg/cm2.
An example of this technology is described in US publication US 2002/0017742 A1, in which an apparatus is illustrated for compression forming of products realised using thermoplastic material, which comprises a plurality of die groups, mobile and movable independently of one another, each of which comprises a female part having a matrix cavity and a male part destined to penetrate the matrix cavity up to defining a forming chamber F of the product.
A pressing and heating station is provided, in which the die group, after having received a batch of material, is inserted between two heating plates of a press; the two plates heat the matrix and the punch of the die by conduction, while the matrix and the punch are subjected to a reciprocal nearing thrust which compresses the batch present in the matrix.
When the plastic material has reached the minimum desired viscosity (in which the material is fluid) and the punch has penetrated into the matrix as far as possible, the material completely fills the forming chamber, giving shape to the product. Then, the die group is freed by the press and transferred into a cooling station, located downstream of the heating station.
This station comprises a second press which provides the die group with a compression thrust equal to the one provided by the first press, and means which cool the die group while it is subjected to the thrust.
The die groups are free, mobile and movable independently of one another, and free to be cyclically inserted in the heating station and subsequently transferred to the cooling station.
A first type of drawback connected with this technology lies in the fact that in order to actuate a correct forming of the product internally of the die, it is necessary to keep the material internally of the mould subjected to a sufficient compression action continuously and constantly over the wholetime in which it is in a not sufficiently-solid fluid state, i.e. from the moment in which it became fluid up to the moment in which, by cooling, it reached a sufficiently solid and stable state.
On the contrary, the apparatus described in US 2002/0017742 after the step of compressing the fluid material in the pressing and heating station, when the die group is transferred into the cooling station, the material inside the die is freed by the action of the press, and the pressure acting on the material internally of the die is annulled.
A second type of drawback of this technology is that essentially the heating and softening (and therefore also the cooling) of the plastic material is done together with the compression of the material internally of the matrix on the part of the punch. This is necessary as it is the die group that is heated (and subsequently cooled) by the press and thus in turn heats, by conduction, the material it comes into contact with; thus for effective transmission of the heat there must be a good contact between the internal surfaces of the die group and the granules of plastic material and among the granules themselves, which contact is greater the greater the pressure exerted by these internal surfaces against the plastic material.
Here too, as with injection, the value of the pressures in play is still relatively high, in the order of 50-100 kg/cm2, which requires equipment that is relatively sturdy and powerful.
Consequently, relevant technical drawbacks of this technology are connected to the heating and cooling of the dies, via which the material placed internally of the die is heated and cooled.
The relatively high compression thrust requires the parts of the die group to be sufficiently thick and sturdy, such as not to deform excessively, and thus female and male parts are required which have a relatively high mass; consequently high quantities of heat have to be transmitted thereto, proportional to the mass, with a consequent high consumption of calories and is proportionally long times for the transmission to take place.
During the cooling stage, relatively large quantities of heat have to be dissipated, which heat is therefore lost.
Finally, machinery is required (the presses) which is relatively powerful and therefore expensive, in order to be able to provide the necessary relatively high compression thrust.
In conclusion, the known compression moulding technology of thermoplastic material following penetration of the punc, into parts of heated dies, requires a relatively high consumption of energy, relatively long execution times and machines (presses) which are relatively powerful and therefore expensive.
Other limits of this application are the practical impossibility of heating the material to a point at which the whole mass thereof is in the liquid state, especially if the mass of the product is relatively high; in practice this technology is especially suitable for the use of expanded plastic in which softening is limited to the periphery of the particles of material without changing the physical state of the internal parts, thus obtaining products having cores that are not very compact, and are indeed non-uniform.
Consequently it is not practically possible to realise products in which a uniform and relatively large mass is required, such as technical articles having high functional quality or articles of high aesthetic quality.